If you ask me, Id have to say another player. I mean, the Corperal Beast is the strongest Monster, but I've seen people hit 99s in Castle Wars before. And in addition to that they can have food, and protection prayers, but most PvP areas are not multi-combat. So Imagine taking on the strongest player in the game, with the best armor and the best weapon, who can take you out in one hit. Or the final battle could be you fighting him/her in multi-combat, but you have no one to help you and your opponent would have a level 230 steel titan also staring you in the face. But I guess it's hard to say who the REAL best fighter in the game would be, but I would say another player.